The recent report that the ALVAC-HIV prime: gp120 protein (AIDSVAX B/E) boost has prevented infection in a small, but significant number of participants, provides hope that a vaccine that can prevent HIV sexual transmission is possible. This study is supported by results with other vaccines evaluated in the SIV:macaque model for AIDS, including our own using a particle mediated epidermal delivery (PMED) DNA vaccine. This vaccine prevented infection in 40% of vaccinates challenged intrarectally with a high dose of a heterologous primary SIV isolate, thereby demonstrating that immune correlates of mucosal protection observed in the SIV:macaque model will mimic mucosal protection in humans. Using a mucosal model developed by our group that enables repetitive sampling of the GALT in vaccinated macaques, we will identify the critical components of the mucosal protective immune response induced by the PMED DNA vaccine. We will test the following hypotheses: (1) A vaccine confers protection not by preventing infection but by blocking virus escape from the mucosal compartment; (2) The host response to viral infection must be present at the mucosal portal of entry to prevent sexual transmission - a condition requiring direct analysis of the mucosal tissues;(3) The ability to appropriately prime and/or recall virus-specific protective immunity in the mucosa are intrinsic propert(ies) of the individual that modulate either virus replication and/or expression of defensins/chemokines comprising the mucosal innate immune system;(4) The induction of virus-specific poly-functional T cells with an effector memory T-cell phenotype recognizing a broad repertoire of epitopes in the mucosal tissues is required for mucosal protection;(5) A PMED DNA vaccine increases the breadth of the T cell responses in the gut by induction of high-avidity T cell clones that localize to the mucosal compartment. We will address these hypotheses in the following aims: Aim 1: Drs. Martinson and Reinhart will directly determine the relationship of host-specific expression of mucosally relevant defensins and chemokines to vaccine-mediated protection by quantifying expression in the GALT and identifying related polymorphisms in protected and unprotected animals. Aim 2: Dr. Fuller will identify adaptive immune correlates of protection in the mucosa and determine their relationship to innate immunity. Aim 3: Dr. Murphey-Corb will determine the tripartite interplay among mucosal virus burden and escape, innate and adaptive responses, and protection induced by the PMED DNA vaccine. Together, these studies will identify novel approaches to enhance vaccine-induced protective immunity and reveal the impact of repetitive low dose (sexual) exposure on vaccine prevention. The objective of this application is to determine the molecular events that occur as a result of sexual exposure to HIV so that we may better understand why some people get infected while others do not. Understanding just what occurs at the site of initial exposure is highly relevant to the development of a vaccine that can prevent sexual transmission of HIV.